It is known that crosslinking of polymers produces polymer networks which have quite different mechanical and physical properties compared to their uncrosslinked linear or branched counterparts. For example, polymer networks can show such unique and highly desirable properties as solvent resistance, high cohesive strength, and elastomeric character.
Crosslinked polymers can be made in situ during formation of the desired polymer product, however, since further processing of the polymer product is often necessary, it is more typical to start from the linear or branched polymer which in the final processing step is cured to a crosslinked material. The curing or crosslinking step is typically activated by moisture, thermal energy, or radiation. The latter has found widespread applications, particularly in the use of ultraviolet light as the radiation source.
In the past, a variety of different materials have been used as crosslinking agents, e.g. polyfunctional acrylates, acetophenoneos, benzophenones, and triazines. The foregoing crosslinking agents, however, possess certain drawbacks which include one or more of the following: high volatility; incompatibility with certain polymer systems; generation of corrosive or toxic by-products; generation of undesirable color; requirement of a separate photoactive compound to initiate the crosslinking reaction; and high sensitivity to oxygen.
Certain polyfunctional benzophenones have been investigated as photocrosslinking agents and/or photosensitizers in various photopolymerizable systems.
JP 54/057560 discloses the use of (bis)benzophenone compounds to photocrosslink non-elastomeric materials--in particular, polyester compositions. When incorporated into polyesters, they impart improved tensile strength and elongation to biaxally stretched films of crosslinked poly(ethylene terephthalate). These films also exhibit enhanced weather, heat, and chemical resistance and improved dimensional stability.
U.S. Pat. No. 4,602,097 (Curtis) discloses the use of (bis)benzophenones as photoinitiators and/or photosensitizers in radiation cured coatings. The poly(ethylene oxide) moiety which separates the terminal benzophenone groups allows the claimed compositions to be more soluble than unsubstituted benzophenones in waterborne coating compositions. The (bis)benzophenone compounds, however, contain hydrogen donating groups, such as the methylenes adjacent to the oxygen atoms of the ether functionalities. These hydrogen donating groups undergo an intramolecular hydrogen abstraction by the photochemically excited (bis)benzophenone structure to provide a lower energy radical which is effective as an initiator, but unsuitable as a photocrosslinker.
U.S. Pat. No. 4,379,201 (Heilmann et al.) is an example of a class of polyacrylic-functional crosslinkers used in the photocuring of (meth)acrylate copolymers. U.S. Pat. Nos. 4,329,384 (Vesley) and 4,330,590 (Vesley) describe a class of fast curing triazine photocrosslinkers which, when mixed with an acrylic monomer and, optionally, a monoethylenically unsaturated monomer, and exposed to UV radiation, forms a crosslinked polyacrylate. The crosslinks formed by both the (meth)acrylates and the triazines in these copolymerizations prevent any further processing, such as hot melt coating, reactive extrusion, or solution coating processes, following the initial photopolymerization.
U.S. Pat. No. 4,737,559 (Kellen et at.) discloses acrylate-functional aromatic ketones (in particular, 4-acryloxybenzophenone (ABP)) which are incorporated with other (meth)acrylate monomers to form pressure-sensitive adhesive copolymers containing pendant benzophenone groups. These benzophenone functional pressure-sensitive adhesive copolymers undergo efficient crosslinking upon exposure to UV light, especially when compared to the use of conventional benzophenone as a photocrosslinker. This patent also specifically states that the disclosed compounds must be free of hydroxy groups in a position ortho to the carbonyl functionality. These hydroxy substituents inhibit free-radical formation and hydrogen abstraction from the acrylate copolymer backbone. However, since these acrylate-functional aromatic ketones are monomers to be copolymerized primarily with other acrylic monomers, they are not useful as a post-polymerization photocrosslinker which may be compounded with previously prepared elastomeric polymers of varying chemical character.
A suitable class of radiation-activatable polyfunctional acetophenone and benzophenone crosslinking agents for elastomers has not been recognized or utilized to date and has been lacking in the industry. It was against this background that a search for such a suitable class of radiation-activatable polyfunctional acetophenone and benzophenone crosslinking agents for elastomers was conducted.